MobiNav

The satellite orbit information retrieved from an almanac is much less accurate than the information retrieved form the ephemeris. If during a startup period, only almanac information is available, (e.g. while the ephemeris still is being downloaded) the receiver still is able to navigate based on almanac orbits.

With almanac navigation enabled, when a new satellite reised and its reception ujst has started, the receiver might use an almanac to use this satellite in the navigation solution until the ephemeris is fully retrieved.

With an almanac only solution, the position will only have an accuracy of a few kilometers. Almanac Navigation allows much faster start up, as there is no need to wait for the completion of the ephemeris download(>18s).

AGPS stands for Aiding/Assisted GPS.

The requireed aiding data:

• ALmanac or ephemeris data
• Position & Time

• GGA – Global positioning system fix data
• GLL – Geographic position – latitude/longitude
• GSA – GNSS DOP and active satellites
• GSV – GNSS satellites in view
• RMC – Recommended minimum specific GNSS data
• VTG – Course over ground and ground speed
• GRS – GNSS range residuals
• GST – GNSS pseudo range error statistics
• TXT – Text messages
• ZDA – Time and Date

GPS works with weak signals. The signal strength on earth is approximately 15dB below the thermal noise floor. In order to design a reliable GPS enabled system, the following parameters have to be considered carefully during the design phase as they may significantly degrade the GPS performance.

1. Antenna shortcommings
• Poor Gain fo the GPS antenna
• Poor directivity(radiatin pattern) of the GPS antenna
• Improper orientatin of the antenna to the sky
• Poor matching between antenna and cable impedance
• Poor noise performance of the receivers input statge or the antenna amplifier
2. Electrical Environment
• Jamming form external signals
• Jamming form signals generated by re receiver itself
3. GPS related effects
• Signal path obstruction by building, foliage, covers, snow,etc
• Multi-path effects
• Satellite constallation and geomatry

Hot start is performed whenever the GPS receiver stil has access to valid ephemeris data and precise time. This is typically the case if the receiver has been shut off for less that 2 hours and the RTC has been running during that time. Furthermore, during the previous session, the receiver must have been navigating (to allow it todecode and store ephemeris data).

In hot start, the receiver can predict the currently visible SVs, and it there fore able to quickly acquire and track the signal. Because ephemeris is already known, there is no need to wait for the ephemeris broadcast to complete.

If external aiding information like ephemeris and/or precise position and time are provided to the receiver, the TTFF may significantly be improved.

Warm start is performed whenever the GPS receiver has only access to valid almanac data, and has not significantly moved since the last valid position calculation. This is typically th ecase, if the receiver has been shut off for more than 2 hours, but still has knowledge oflast position, time and almanac. This allows it to predict the current visible SCs. However, since ephemeris data is not available oroutdated, the receiver needs to wait for the ephemeris broadcast to complet.

In cold start, the GPS receiver has no knowledge of its last position or time. This is the case whenthe RTC has not been running; no valid ephemeris data or almanac data is available.